The mechanisms involved in the development of radiation nephritis are poorly understood, particularly with respect to man. Most studies have employed whole body exposure of rodents. Those studies, as well as experiments in dogs utilizing in-situ renal irradiation, fail to correlate altered function with abnormal structure. Renal function in man is dissimilar to that of rodents and dogs, and whole body exposure neither simulates the therapeutic situation nor permits evaluation of function in the absence of compensatory change in function seen in bilateral renal disease. In an attempt to circumvent these problems we propose to: 1) Evaluate radiation nephritis in a nonhuman primate (Macaca speciosa); 2) Correlate altered renal function with abnormal structure as a function of radiation dose and time post-exposure; 3) Administer radiation within the therapeutic range to one kidney; 4) Divide the urinary bladder according to the method of Bricker, thereby making it possible to perform split function studies, and to compare the function of the irradiated kidney with the contralateral nonexposed kidney; and 5) Finally, to define specific functional and/or morphologic change which will anticipate irreversible damage. Such abnormalities would have clear prognostic implications for the therapist.